There is growing pressure for stationary producers of greenhouse gases to dramatically reduce their atmospheric emissions. Of particular concern is the emission of carbon dioxide (CO2) into the atmosphere. One method of reducing atmospheric CO2 emissions is through its capture at a point source and subsequent storage in geological or other reservoirs.
The process for capturing CO2 from power station and other combustion device flue gases is termed post combustion capture (PCC). The most mature commercially available technology for PCC is solvent-based chemical absorption/release of CO2. When the widespread rollout of PCC technology is realised, enormous quantities of solvent such as ammonia and amine will be required. To put this in perspective, a typical 2.4 GW generator burning pulverised black coal produces approx. 30-50 tonnes CO2/min, or 680 kmol/min. Clearly, the quantity of solvent that will be used cyclically in a PCC plant of this magnitude is unprecedented. The potential environmental impacts of solvents and solvent degradation products (produced via oxidative and thermal processes) needs consideration, especially as release to the local environment through solvent slippage at this scale may be inevitable.
The most attractive physical solvents for CO2 capture are those having such properties as high thermal stability, extremely low vapour pressures, non-flammability, and non-toxicity. Such materials have the potential to capture CO2 with minimal solvent loss in the gas stream.
The use of solutions of ammonia for removing CO2 from flue gas streams is attractive from a chemistry perspective, with a number of important advantages relative to systems that employ monoethanolamine (MEA) or other amines as the solvent, long-known for recovering CO2 from gas mixtures. These advantages include
1. SOx and NOx can be absorbed with the possibility of advantageously selling the spent solvent solution as a fertiliser (SOx and NOx degrade amine solvents).
2. Ammonia is a low cost chemical, in widespread commercial use.
3. The overall energy required for such a process is projected to be around 40% of that required for MEA systems.
For the ammonia process, the solvent solution consists of ammonium, bicarbonate ions and to a lesser extent, carbonate, in equilibrium with dissolved ammonia (aqueous), and dissolved CO2 (aqueous). In the absorber, water and ammonia react with CO2 (aqueous) to form bicarbonate ions or ammonium carbamate ions, with the reaction reversed in the stripper by the application of energy. The relevant aqueous phase reactions can be summarized by the following overall equations:CO2+H2O+NH3HCO3−+NH4+  (eqn. 1)CO2+2NH3NH2COO−+NH4+  (eqn. 2)HCO3−+NH3CO32−+NH4+  (eqn. 3)CO32−+H2O+CO22HCO3−  (eqn. 4)
Ammonia is a light molecule (molecular weight=17 dalton) which is a gas at temperatures above −30 degrees Celsius (at atmospheric pressure), and it is readily dissolved in water. Sparging aqueous ammonia solutions with a gas stream (such as in a post-combustion capture process) will result in ammonia leaving the solution and entering the gas phase. The amount of free ammonia in the gas phase exiting the absorber is proportional to the amount of aqueous ammonia, which is controlled by solution chemistry and thermodynamic factors such as the temperature: higher temperatures increase the amount of ammonia in the gas phase.
The major concern with the ammonia process has been ammonia loss (or “slip”) associated with both the absorber and the stripper. One possible approach to mitigating ammonia slip from capture plants is the use of additives to elevate the aqueous ammonia boiling temperature, thus decreasing ammonia loss via a salt- or azeotropic effect.
It is an object of the present invention to overcome or at least alleviate one or more of the problems associated with the use of ammonia as a CO2 capture solvent.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.